A Mach-Zehnder type modulator produced with guided optics, is described in R. Keil and F. Auracher's article (Siemens Forsh-u. Entwickl-Ber. Bd. 9 (1980) Nr 1) "Mach-Wehnder Waveguide Modulators in Ti-Diffused LiNbO.sub.3 ". It consists of:
(a) a platelet of transparent crystalline material whose optical index varies by electro-optical effect under the action of an electrical field,
(b) integrated optical guides, these guides being formed in one face of this platelet by permanent localized increases of the optical index resulting from doping of the material, these guides being monomode both as to their width and to their thickness and format,
(c) an input guide receiving and conducting a light with a space coherence,
(d) an input optical junction in Y to separate the light of the input guide into two beams of the same intensity,
(e) two interferometric arms of similar length to conduct the two beams thus separated, these two arms extending parallel to each other in a longitudinal direction,
(f) an optical output junction in Y to recombine these two beams from the two arms,
(g) and an output guide receiving the light thus recombined and transmitting it on output from the modulator, the length of this guide being sufficient for it to only transmit its fundamental mode,
(h) the modulator also containing electrodes formed on the said face of the platelet, associated to the two interferometric arms, and receiving an electrical modulation signal to apply to the material of its two arms, two electrical fields varying the optical length of these two arms in opposite directions and thus modulating the intensity of the light transmitted by the output guide, these electrodes extending along the said longitudinal direction over practically the whole length of the arms to increase the sensitivity of the modulator.
The optical guide (or integrated optics) obtains very high speed modulators (large pass-bands) with reasonable control voltages (a few volts). But the known modulators do not have as great a sensitivity as would be sometimes required, i.e., the degree of light modulation obtained with a low electrical control voltage is insufficient. It can be observed that, to a certain extent, there is incompatability between the sensitivity and the speed; the more modulator needs to be sensitive, the longer should be the electrodes and the higher the electrical capacilance entailing a reduction in the pass-band.
The most suitable material to produce an electro-optical modulator is lithium niobate (LiNbO.sub.3) because it enables the production of good optical guides and is highly electro-optical. The light guides are obtained on a face of a monocrystalline platelet, this face being perpendicular to crystalline axis 3 also referred to as axis Z, at a polarized light is used along this axis, i.e., the optical electrical field is parallel to this axis. The electrical modulation field applied is also parallel to this axis.
Interferometers of this type are known for example through document FR-A No. 2 457 505, and by a paper read to the second European Conference on Integrated Optics (17-18 October 1983 Florence, IEE Number 227 "Linear Mach-WehnderInterferometers in LiNbO.sub.3 for Electromagnetic Field Sensing" C. H. Bulmer, R. P. Loeller and W. K. Burns, Naval Research Laboratory, Washington, D. C. 20375, USA).
In these known modulators one of the electrodes associated with each of the arms is an overlapping electrode which overlaps this arm so that the electrical field applied to this arm is practically perpendicular to the said face of the platelet, the other electrodes being lateral electrodes formed at a lateral distance from these arms.
Moreover, to increase the sensitivity, these documents indicate that the electrodes are extended as far as their input and output junction areas in Y.
It is moreover known, in a similar technical field, through an article by Al Ferness and others "High-Speed, Low Loss, Low-Drive-Power Travelling-Wave Optical Modulator for Lambda=1.32 Micron" (Electronic Letters, Apr. 12, 1984, Vol. 20 No. 8 Page 354) how to lower the control power of a directional optical coupler to five volts by reducing the size of the guided mode in an optical guide, i.e., the width of this guide. The aim is to reduce the spacing between two parallel guides overlapped by two control electrodes, to obtain the correct electrical control field by applying a reduced electric voltage between these electrodes.
The sensitivities obtained nevertheless remain less than might be desired, notably in the case in which a modulator is used to obtain a hydrophone in association with a piezoelectric tranducer, or for that of a device measuring low electric voltages, for example of around a few tenths of a volt.
The aim of this invention is to produce an interferential electrooptical modulator, offering increased sensitivity whilst preserving a large passband at a reasonable cost price.